Bug Description

In Greece, the default system layout is "us,gr":
$ grep XKBLAYOUT /etc/default/keyboard
XKBLAYOUT="us,gr"

For new users that don't have .dmrc or accountsservice entries and are supposed to get the system defaults, LightDM assumes their keyboard layout is "us".
So they can't switch languages in the greeter and they can't input national characters until they login, go to gnome settings and manually set the keyboard layout.

Here's a list of the affected countries. All these people can't type national characters unless they know how to go to gnome control center and add their keyboard layout and their preferred switching method.

This is a serious regression, it used to work fine in gdm and in gdm2.

I can confirm that this is a pretty big problem if you have two users who use different keyboard layouts. One of them simply cannot really enter his password and has to learn how to type it on a different layout.

2017-06: Quite a long time for such a bug to survive.
As Anatoly said, it is worse on 16.10 .
If you lock the session with wrong layout, you are doomed.
Is there any hint where we should search for a solution?
Do you have any place in code to try to debug?